1. Field of the Invention
The present invention relates generally to the fabrication of magnetic heads for hard disk drives and more particularly to magnetic heads having current perpendicular to plane (CPP) tunnel junction read sensors with at least one ELG and a partially milled stripe height.
2. Description of the Prior Art
A computer disk drive stores and retrieves data by positioning a magnetic read/write head over a rotating magnetic data storage disk. The magnetic head reads data from or writes data to concentric data tracks defined on surface of the disks. The head is fabricated in a structure called a “slider” and the slider flies above the surface of the disk on a thin cushion of air, where the surface of the slider which faces the disk is called an Air Bearing Surface (ABS). The ABS is typically fabricated utilizing lapping techniques that are controlled by electronic lapping guides (ELGs).
Various read head sensors are known in the art and some recent read head structures use a tunnel junction sensor, also known as a “tunnel valve” for reading the magnetic data bit signals from the rotating magnetic data storage disk. The tunnel junction sensor typically includes a nonmagnetic tunnel barrier layer sandwiched between a pinned magnetic layer and a free magnetic layer. The pinned layer in turn is fabricated on an antiferromagnetic (AFM) pinning layer which fixes the magnetic moment of the pinned layer at an angle of 90 degrees to the air bearing surface (ABS). The magnetic moment of the free layer is free to rotate from a quiescent or zero bias point position in response to magnetic field signals from magnetic data bits written on the rotating magnetic disk. The tunnel junction sensor layers are typically disposed between first and second magnetic shield layers, where these first and second shield layers also serve as first and second electrical lead layers for conducting a sensor current through the device. The tunnel junction sensor is thus configured to conduct sensor current perpendicular to the planes (CPP) of the film layers of the sensor, as opposed to previously developed sensors where the sensor current is directed in the planes (CIP) or parallel to film layers of the sensor. The CPP configuration is attracting more attention recently, as it apparently can be made to be more sensitive than the CIP configuration, and thus is more useful in higher data density recording devices.
The read width and the stripe height of the sensor are significant well known parameters that refer to the width of the read head sensor stack and the height dimension of the sensor stack perpendicular to the ABS. Both of these dimensions are very important to the operating characteristics of the read head and they are typically defined using ion milling techniques. A problem that can occur in the fabrication of the prior art CPP sensors is that the ion milling can damage the tunnel barrier layer edges, which can cause unwanted reduction of electrical resistance and even electrical shorting of the tunnel barrier layer. This problem can be solved by the partial milling of the sensor down only through the tunnel barrier layer. However, a second related problem can then arise where the electronic lapping guides (ELGs) are not properly fabricated. That is, where the ELGs are fabricated in the same milling steps as the sensor, the partial milling of the sensor must nevertheless be sufficient to mill completely through a layer of ELG material in order to properly shape it. Specifically, where the back edge of an ELG is created by milling in the same step in which the stripe height of the sensor is milled, the partial milling step (downwardly through the tunnel barrier layer) must be sufficiently robust to mill entirely through the ELG material. In this regard, the selection of ELG material and the thickness of the ELG material are significant parameters that must be properly chosen.
Additionally, in the prior art, the ELG material layer and the sensor layers are not coplanar, and the optimum optical focusing that is utilized to accurately create the milling mask for one layer, such as the tunnel barrier layer, will not be optimum for fabricating the mask for the non-coplanar ELG layer. That is, the optical focusing for the fabrication of an accurate milling mask for the stripe height results in less than fully accurate focusing for the fabrication of the ELG mask because they are located at different focal planes. As a result, there is unwanted variation in the optical focusing for fabricating the ELG mask, which results in unwanted variation in the size of the ELGs. This variation can become significant across the surface of a wafer where there already exists some unwanted variation in the sharpness of optical focusing at different locations across the surface of the wafer, as is well known to those skilled in the art. The significance of variations in the ELG fabrication is that the air bearing surface (ABS) of the individual heads is determined by the electrical properties of the ELGs, and where the size of the ELGs varies across the surface of the wafer, the location of the ABS will similarly vary for magnetic heads disposed at different locations on the wafer. As a result of the differing locations of the ABS of magnetic heads across the surface of the wafer, the sensors of the different magnetic heads will likewise be fabricated with differing properties. Quality control and manufacturing process throughput are adversely affected where the magnetic heads that are fabricated on a single wafer substrate are created with differing properties due to variations in the location of the ABS that is due to variations in the fabrication of the ELGs. Thus, there is a need for a method of sensor fabrication which eliminates damage to the tunnel barrier layer when ion milling is used to shape sensor material stacks, and which accurately shapes the ELGs.